Circuits and circuit designs are typically based on electrical and electronic components and properties and are useful for a variety of functions. An electrical circuit is an interconnection of electrical elements, such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources, and switches, and when it also contains active electronic components is known as an electronic circuit. Electronic circuits can usually be categorized as analog, digital or mixed-signal (a combination of analog and digital) electronic circuits. The basic units of analog circuits are passive (resistors, capacitors, inductors, and memristors) and active (independent power sources and dependent power sources). Components such as transistors may be represented by a model containing passive components and dependent sources. In digital electronic circuits, electric signals take on discrete values, which are not dependent upon time, to represent logical and numeric values. These values represent the information that is being processed. The transistor is one of the primary components used in discrete circuits, and combinations of these can be used to create logic gates. These logic gates can then be used in combination to create a desired output from an input.
In contrast, while some biological circuits have been developed, the utility of these circuits has been minimal, and it has been difficult to replicate the versatility and flexibility of standard electronic circuits. Many challenges remain in advancing synthetic biology from low-level gene circuitry to higher-order networks. Controlling the state of cells is a difficult but important task in biotechnology. For example, controlling transcriptional activity in cells currently relies on either constitutive promoters which are hardwired to have distinct activities or by using transcriptional activators or repressors which can be tuned by the application of inducer molecules. However, inducer molecules are expensive, can be difficult to control in complex environments used in biotechnology, and can be toxic. Constitutive promoters cannot be easily shut off and therefore can be difficult to use when there are toxic products that are being expressed. Furthermore, constitutive promoters severely limit the flexibility of biological systems to adapt to different conditions.